1. Field of the Invention
The present invention relates generally to a semiconductor device formed in a thin film monocrystalline semiconductor layer prepared on the surface of a substrate and a method for making same. More particularly, the present invention is directed to a process for the preparation of integrated circuit devices in a thin film of a monocrystalline semiconductor material supported on the surface of a substrate.
Semiconductor devices are most usually formed by diffusion and/or ion implantation of doping ions into a monocrystalline body of semiconductor material in combination with intricate and precise masking and etching steps. The semiconductor is usually a wafer of monocrystalline silicon cut from a cylinder of monocrystalline silicon which is drawn from a bath of molten silicon. Because of physical strength requirements a wafer of silicon which is many times thicker than is required for formation of the semiconductor devices is produced. The thickness required is related to the diameter, i.e., 0.203 mm for a 3.175 cm diameter, 0.330 mm for a 5.715 cm diameter and 2.235 mm for a 8.255 cm diameter wafer. The present invention is directed to semiconductor devices formed in a film of monocrystalline silicon supported on a substrate. In a preferred embodiment the present invention is directed to a process for the preparation of integrated circuit devices in a thin film of a monocrystalline semiconductor material formed from a thin film of polycrystalline semiconductor material by means of high energy annealing, such as by use of a laser beam.
2. Prior Art
Laser annealing is a rapidly emerging technology with great potential for use in semiconductor processing. A major advantage of laser annealing is that laser beams can heat localized regions to a high temperature for a short duration. Temperatures in excess of the melting temperature of the material can be attained. Laser annealing has primarily been proposed for use in healing the damage caused by ion implantation during the fabrication of resistors and transistors. Another area proposed for use of laser annealing is the fabrication of transistors and integrated circuits using silicon on sapphire technology.
Ion implantation is a technique that provides precise dosages of doping ions and the profile control needed for solid state device fabrication. The implanted atoms, however, produce considerable disruption of the semiconductor crystal lattice in the form of dislocations, stacking faults and dislocation loops which act as traps for charge carriers. Many of the implanted doping atoms are located at interstitial sites within the lattice. Consequently, they are not electrically active until additional processing moves them from the interstitial site into a substitutional position. This movement of atoms is most usually accomplished by heating the semiconductor wafer in a high temperature furnace. The technique of thermal annealing is only partially effective, however, and additional problems are introduced during the relatively long period required for temperature annealing.
Both pulse and CW-laser annealing of ion implantation damage have been found to restore crystallinity to the host lattice and to produce high electrical activation of doping atoms in silicon. Pulse-laser annealing provides a somewhat higher degree of crystallinity and electrical activation of implanted and deposited layers, but the pulse-laser technique results in some significant redistribution of the doping atom profile. CW-laser annealing provides a high degree of electrical activation without disturbing the implanted doping atom profile.
Experiments conducted by Hughes Research Laboratories, reported in Vacuum Technology-Industrial Research/Development, "Laser Annealing", November, 1979, pp. 141-152, indicate that healing of ion implant induced damage by laser annealing is the result of epitaxial growth through either solid-phase epitaxy of liquid-phase epitaxy, depending on the type of laser used. Such epitaxial growth induced by laser annealing is well known when the disrupted crystalline material is in contact with a monocrystalline source. As pointed out in the article, the time available for crystal regrowth during pulse-laser annealing is limited by heat conduction to approximately 1 microsecond. The extremely short time available for crystal regrowth has been thought to be a limitation on the use of laser annealing to form a crystalline material from a monocrystalline source. It has been thought that laser crystallization of thin-film amorphous silicon is by means of epitaxial silicon growth. As described in an article by J. C. Beam et al. "Epitaxial Laser Crystallization of Thin-Film Amorphous Silicon", Appl. Phys. Lett., Vol. 33, No. 3, Aug. 1, 1978, pp. 227-230, vapor-deposited amorphous silicon films of 4000 A thickness have been epitaxially crystallized when the films are located on the surface of monocrystalline substrates by pulse-laser radiation. The concept of epitaxial growth requires that the amorphous silicon to be recrystallized into the form of monocrystalline silicon be in contact with a monocrystalline source.
Some attempts have been made to recrystallize amorphous polycrystalline silicon thin film layers on the surface of amorphous substrates (see A. Gat et al., "Appl. Phys. Lett." 33,775 (1978). In the study of Gat et al. a 0.4 micrometer-thick continuous polycrystalline film having an initial average grain size of 500A was prepared by low pressure chemical vapor deposition on a Si.sub.3 N.sub.4 substrate. The film was held at a temperature of 350.degree. C. during laser annealing. The annealing was accomplished with an Ar laser having an output of 11 W focused into a 40 micrometer spot and scanned across the film at a rate of 12 cm/sec. Under these conditions columnar crystallites were found to extend completely through the film to the nitride interface. The crystallites had a typical surface area of 2 micrometers by 10 micrometers and were found to develop at an angle to the direction of the scan, producing a chevron-like structure. The crystallites displayed a number of crystal orientations with no indication of a preferred orientation. Such columnar crystallites, of course, do not constitute the formation of a monocrystalline silicon material useful for the preparation of semiconductor devices.
The ability to form thin layers of monocrystalline silicon on the surface of amorphous substrates is of immense importance in furthering the art of large scale integrated semiconductor devices. Such ability to form thin layers of monocrystalline semiconductor material can be used to provide multilayer semiconductor devices, thus drastically increasing the density of the device.
Accordingly, it is a principal object of the present invention to provide a process for the preparation of integrated circuit devices in a thin film of a monocrystalline semiconductor material supported on the surface of a substrate.
Another object of the present invention is to provide a thin film of a monocrystalline semiconductor material on the surface of an amorphous substrate having integrated circuit devices formed therein or thereon.
A further object of the present invention is to provide integrated circuit devices in a thin film of a monocrystalline semiconductor formed on the surface of a substrate by laser annealing.